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An artificial discharge of water (3 .o m 3 /sec), over a 48 h period, from an impoundment into the R . Wye did not substantially affect water temperature or concentrations of dissolved oxygen and suspended solids at a site 16 km below the impoundment . However, the load of suspended material on the second day of the release was about to times greater than the pre-release load . The total number of drifting macroinvertebrates on the first and second days of the release were about 7 and 3 times greater than the number on the day preceding the release . The initial increase in flow at 15 .oo h resulted in an immediate increase in the number of drifting larvae of Rheotanytarsus, a tubicolous chironomid . Subsequently there was an enhanced night-time increase in the total number of drifting invertebrates, particularly the mayfly, Ephemerella ignita (Poda), and this also occurred on the second night of the release . Increases in the number of drifting Rheotanytarsus and Ephemerella, the most abundant invertebrates, resulted in increases in drift density .

SUMMARY. Surveys of the Wye catchment in March and September 1975, and July and September 1976 and 1977, yielded 227 macro‐invertebrate taxa, 77 of which were Chironomidae. Total macro‐invertebrate density ranged from 520 to 22020 m−2. Total density estimates in July 1976. during a period of severe drought, were significantly higher than in July 1977 but no differences were established between estimates on other comparable dates in 1975, 1976 and 1977. The total number of taxa and total macro‐invertebrates collected from sites with total dissolved solids concentrations less than 50 mg I−1 were significantly lower than at sites with higher concentrations. In general, upstream sites were dominated numerically by Plecoptera in March; Ephemeroptera, Trichoptera and Diptera were relatively more abundant during the summer months. Sites lower in the catchment were more variable in composition with Oligochaeta, Ephemeroptera, Trichoptera and Diptera all abundant at certain sites and times. The densities of Ecdyonuridae (Ephemeroptera). Hydropsychidae (Trichoptera) and Elminthidae (Coleoptera) in the impounded R. Elan, a tributary of the R. Wye, were significantly lower than at nearby sites on the Wye although no differences in total macro‐invertebrate density were established. Average linkage clustering of Spearman rank correlation coefficients between selected taxa allowed the comparison of taxa groups with contemporary site groups by nodal analysis. Six major taxa groups were established and these characterized five site groupings.

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